Soap is a higher fatty acid alkali metal salt and conventionally has been used in detergents for the body (for the skin) such as the face and hands. As a washing material, soap not only produces a unique fresh feeling after washing but also has good biodegradability and is easy on the environment.
Various such body detergents using the soap have been developed. For example, solid-type, powder-type, liquid-type, and paste-type body detergents have been widely used for years. Of these, powder-type body detergents dissolve more readily than solid-type body detergents, and thus are superior in that they readily dissolve even in cold water and in that only the amount necessary need by discharged, for example. Further, compared to liquid-type and paste-type body detergents, powder-type body detergents are less likely to leak when transported and are less prone to dirty the container when discharged, and thus are easy to use. Consequently, some benefits to powder-type body detergents (soap) are that the sink can be kept clean when they are used, and that they are produced in a form that is highly portable, for example.
Powder soaps normally are produced by preparing an aqueous soap solution, drying the solution by a spray drying method or the like, then adding binder such as water and a non-ionic surfactant, and grinding this into powder with a high-speed mixer, a Henschel mixer, or a Loedige mixer, for example. In order to increasing the solubility, the water content of powder soaps prepared in this manner normally is kept to about 7 to 15%. However, such powder soaps have poor flowability, and blocking readily occurs. Therefore, such powder soaps are preferable only in cases where they are used as clothing detergent that is required high solubility and may be taken out with a measuring cup.
In general, powder-type body detergents are packed into a vessel with a small discharge opening, and when used, it is required to simply discharge a suitable amount of the detergents from the discharge opening. Powder soap that serves as a body detergent therefore is required to have excellent flowability and not be prone to blocking which adversely affects flowability, but to date, a powder soap that has sufficient flowability and anti-blocking property has yet to be obtained.
The fatty acid primarily used as the raw material for soaps is derived from palm oil, coconut oil, and palm kernel, and contains 30 to 40 wt % of unsaturated fatty acid. For example, Japanese Laid-Open Patent Publication No. 8-3597 discloses a powder soap whose main component is sodium oleate, which is an unsaturated fatty acid, and that readily dissolves in cold water. However, although the powder soap obtained by using the unsaturated fatty acid as the raw material has excellent solubility, there is the problem that it may change color or produce foul smells. To solve the problems of changes in color and the production of foul smells, for example, Japanese Laid-Open Patent Publication No. 2000-160196 discloses a soap that uses a saturated fatty acid having 12 to 16 carbon atoms. However, this soap is a solid soap, and when turned into powder, it has poor flowability and blocking occurs. Further, Japanese Laid-Open Patent Publication No. 2004-189621 discloses a powder preparation for face cleansing that contains a sodium saturated fatty acid as a main component and vitamin E and porous silicon dioxide. This powder preparation has good washability and foamability, and leaves an excellent skin feel. Additionally, blocking of the powder preparation is prevented by an addition of large amount of porous silicon dioxide. However, flowability and anti-blocking property of the powder preparation are insufficient. Powder soap that has sufficient flowability and anti-blocking property is desirable.
It is an object of the present invention to provide a powder soap that has good flowability. It is a further object of the present invention to provide a powder soap for the body (for the skin) such as the face and hands that has good solubility, is not prone to blocking, has excellent foamability and foam retention, provides a good feeling of the skin after washing and has excellent storage stability.